Before setting up a data link to one or more base stations (BTS), every mobile radio receiver must synchronize itself to the transmit and receive clock. In CDMA transmission methods, in particular, the most accurate possible and early (early means at the beginning of the initial synchronization process, if possible) frequency synchronization is necessary since frequency deviations in the receive clock cause a significant degradation in the demodulation of the spread spectrum coded signals.
One of the essential performance criteria of a synchronization device are the detection probabilities, the mean acquisition time, the so-called false alarm rate and time needed by the system for detecting a “negative detection”. To ensure optimum synchronization performance, the transmitter and receiver must operate frequency-synchronously as accurately as possible. If the mobile radio receiver is already synchronized with a base station, the frequency with respect to new cells to be found is negligible as a rule (e.g. <0.1 ppm in the case of UMTS). For initial synchronization attempts (e.g. after “power on”), in contrast, considerable frequency offsets must be expected between transmitter (BTS) and receiver (mobile terminals) (e.g. 3 ppm). A correction of the frequency offset before or during a very early phase of a synchronization procedure is therefore desirable and advantageous.
In spread spectrum code modulated transmission systems, frequency synchronization and frequency tracking are performed, as a rule, on a basis of received symbols of a pilot channel. For this purpose the pilot channel is demodulated (de-spread) and the phase difference between successive received signals is determined. The method is “state of the art” and can be looked up in detail in the relevant technical literature. This standard method has some disadvantages. On the one hand, it is necessary that the reference channel (pilot channel) transmits a sufficiently long sequence of contiguous successive symbols. In the UMTS system, for example, not all channels meet this criterion (e.g. the synchronization channel SCH only transmits one pilot symbol per UMTS slot, i.e. a symbol period of about 0.66 ms). On the other hand, the demodulation sequence (especially the spread spectrum sequence in this case) of the pilot channel is not known, as a rule, in the receiver during the initial synchronization procedure. It is only determined during the initial synchronization procedure or cell detection, respectively. Thus, a standard method cannot perform a correction of the frequency offset in this mode of operation. In consequence, all stages of an initial synchronization procedure (an initial synchronization consists, as a rule, of a number of stages which use/demodulate different channels) must be performed with a frequency offset which, as a rule, is high.